Block copolymers have already been known for a long time. They have proven themselves for a multiplicity of different applications and have become established accordingly. Thus, for example, rubbers (e.g. natural rubber or styrene-butadiene rubber) or styrene block copolymers (SIS, SBS) are used as a basis for producing self-adhesive tapes.
Numerous self-adhesive tapes comprise as an elastomer component polyacrylates, whose technology has already been known for more than 40 years. These adhesive tapes have a multiplicity of advantages over rubber-based adhesive tapes.
Their qualities include excellent UV and light stability, high resistance to thermooxidative aging, polarity adjustable in accordance with the respective requirement, and, typically, a water-clear transparency. Polyacrylate pressure-sensitive adhesives, moreover, by virtue of the possibility they generally afford, and which is generally taken advantage of, of crosslinking the polymer chains, possess high cohesion even at relatively high service temperatures, and hence their adhesive bonds possess a high temperature resistance. A further advantage is that polyacrylate pressure-sensitive adhesives are tacky as they are, i.e. without additional additives such as tackifier resins or plasticizers, for example.
In order to meet the increasing requirements imposed from industry on polyacrylate pressure-sensitive adhesives the recent past has seen the specific development of polymerization processes for controlling the molecular weight distribution (K. Matyjaszewski (ed.), Controlled/Living Radical Polymerization, ACS Symposium Series, No. 768, ACS, Washington, 2000). With such polymerization processes it is possible with advantage to utilize synthesized polyacrylates for systems including pressure-sensitive adhesives which can be coated from the melt (DE 100 30 217; DE 100 36 801; DE 101 49 084). The improvements, however, are slight.
Conventional approaches to controlling the properties of polyacrylate pressure-sensitive adhesives include the choice of the nature and amount of the comonomers employed, the adjustment of molar mass and molar mass distribution of the polymers, and the nature and extent of the crosslinking of the polymers. The aforementioned variables allow targeted and precise control of the profile of adhesive properties.
Another path to improved products is that of the targeted synthesis of block copolymers (N. Hadjichriszichis, S. Pispar, G. A. Floudas, Block Copolymers, Wiley, N.Y., 2003). As a result of chemical coupling of thermodynamically incompatible polymer blocks such block copolymers exhibit microphase separation: that is, thermodynamically compatible polymer blocks associate, whereas thermodynamically incompatible blocks segregate into spatially separate regions, but without any macroscopic phase separation occurring. This results, depending on composition, in phases differing in structure. Block copolymers currently utilized in pressure-sensitive adhesives typically possess two or more polymer blocks having a high softening temperature (also referred to as hard block below; realized by means of a correspondingly high glass transition temperature or a correspondingly high crystallite melting temperature) and at least one block of low softening temperature (also referred to below as soft block). The composition in systems employed to date has been chosen such that the phase formed by the soft blocks constitutes a continuous matrix within the pressure-sensitive adhesive, which is the prerequisite for the pressure-sensitive adhesion properties. The polymer blocks which soften at a high temperature associate or segregate to form phase regions (domains) which are typically approximately globular, which are present in dispersion in the continuous matrix of the soft phase, and which below their glass transition temperature or crystallite melting temperature function as physical crosslinking points (G. Holden, N. R. Legge, R. P. Quirk, H. E. Schroeder (Ed.), Thermoplastic Elastomers, 2nd ed., 1996, C. Hanser Verlag, Munich). Advantages of pressure-sensitive adhesives based on such block copolymers include, for example, the possibility of realizing very high shear strengths.
The melt coatability of the systems, however, is poor, owing to the pronounced phase separation described above. Therefore further developments were undertaken in order to improve the melting readiness of block copolymers. One route taken, for example, is that of the star polymers, as they are known, which owing to their three-dimensionally branched structure have a lower flow viscosity than linear systems of equal molar mass. Many of these block copolymers have already been described. Examples can be found in Japanese Official Patent Provisional Publication (Kokai) No. Showa 63-132914, in Japanese Official Patent Provisional Publication (Kokai) No. Heisei 3-190911 or in Japanese Patent Publication (Kohyo) No. Heisei 5-500827. Another approach has been described in EP 0 686 653. It gives a description of polyvalent mercaptans as connecting elements in star-shaped block copolymers. These materials can be coated very efficiently from the melt. A general disadvantage of these systems, however, is the relatively poor solvent resistance, since the network present is only a physical one.
It is an object of the invention to provide block copolymers which can be coated easily, in other words at low temperatures, but which can then be crosslinked so that the crosslinked products can be used as pressure-sensitive adhesives (PSAs). In the course of the coating operation decomposition processes ought largely to be avoided. The block copolymers ought advantageously to be coatable free from orientation.
A further object of the invention is to offer a process for preparing pressure-sensitive adhesives based on such block copolymers.
In particular it could not have been expected that block copolymers of the type specified below would be coatable easily, i.e. at low, gentle temperatures, and yet after an operation of crosslinking by UV radiation would have the shear strengths desired for PSAs.